In recent years, so-called electric vehicles, which are driven by electric power charged in a vehicle-mounted battery, are becoming widespread. The electric vehicles have unique difficulties such as electric power insufficiency during their travel and battery deterioration due to overcharging.
An electric power supply system disclosed in JP 2005-210843A enables an own vehicle to receive electric power supplied from the outside, such as a roadside power supply apparatus or another vehicle, via an electromagnetic wave or the like when the own vehicle has insufficient electric power, and enables the own vehicle to supply electric power to the outside of the own vehicle when it has extra electric power. More specifically, when, for instance, the amount of charge in a battery of the own vehicle is small, the electric power supply system causes a power supply apparatus mounted in the own vehicle to transmit an electric power request signal to a nearby roadside power supply apparatus or another vehicle. If, in such an instance, the own vehicle is near a roadside power supply apparatus or another vehicle capable of transmitting electric power, the roadside power supply apparatus or the other vehicle transmits electric power in response to the electric power request signal from the own vehicle. As a result, the own vehicle can receive the electric power transmitted from the roadside power supply apparatus or the other vehicle and increase the amount of charge in the battery.
An electric power supply system described in JP 2005-168085A includes a vehicle that transmits electric power, a vehicle that receives electric power, and an information management center that manages information exchanges with these vehicles. The information management center manages the information about individual vehicles. If multiple vehicles may transmit electric power and multiple vehicles may receive the electric power, the information management center selects an electric power-transmitting vehicle and an electric power-receiving vehicle under conditions that, for example, the relative distance between them is the shortest.
In the electric power supply system, when the amount of charge in its battery is small, the own vehicle transmits an electric power request signal. However, if vehicles within the reach of the electric power request signal cannot supply electric power, the own vehicle cannot receive electric power from another vehicle. In reality, it is difficult to always expect that the own vehicle, which is short of battery power, will timely encounter another vehicle capable of supply electric power.
Meanwhile, in the electric power supply system, the information management center merely selects the combination of an electric power-transmitting vehicle and an electric power-receiving vehicle under conditions that the relative distance between them is the shortest. Therefore, vehicles traveling in opposite directions may be selected as the electric power-transmitting vehicle and the electric power-receiving vehicle simply because they are presently located a short distance from each other. In such an instance, in order to transmit or receive electric power, at least one of the selected vehicles needs to significantly deviate from a route to its original destination.